Dampening systems of the type disclosed in U.S. Pat. No. 3,168,037 and U.S. Pat. No. 3,343,484 to Harold P. Dahlgren have offered significant improvements over dampening systems previously employed.
Such systems have included two rollers disposed in pressure indented relation, one of the rollers having a relatively hard hydrophilic surface and the other roller having a smooth resilient surface. In the preferred embodiment illustrated in the drawings of the aforementioned patents the transfer and metering rollers were geared together to travel at substantially equal surface speeds and were driven by a variable speed drive means for metering a film of dampening fluid through a nip between the rollers and for transferring the film of dampening fluid to the lithographic printing system.
The thickness of the film of dampening fluid delivered to the surface of the transfer roller moving out of the nip between the metering roller and transfer roller was controlled primarily by adjustment of pressure between adjacent surfaces of the metering and transfer rollers.
The rate of the metered film of dampening fluid, carried on the surface of the transfer roller and delivered to the lithographic printing system, was controlled by the variable speed drive means. Briefly stated, the theory of operation was that given a film of predetermined thickness the quantity of dampening fluid delivered was directly related to the speed of the film. In other words, to reduce the rate at which dampening fluid was delivered to the lithographic system, the transfer roller speed could be reduced; and, to increase the quantity of dampening fluid, the speed of the transfer roller could be increased. However, such results follow only so long as the speed differential between the transfer roller and the ink coated form roller was not excessive.
Excessive slippage resulted in application of hydraulic forces in the nip between the transfer roller and the applicator roller which caused excessive emulsification of the distinct films of ink and dampening fluid. Excessive emulsification of ink and dampening fluid at the nip between the transfer roller and form roller resulted in transfer of the emulsion by the surface of the transfer roller to the surface of the resilient metering roller which was not hydrophilic. Build-up of ink on the surface of the metering roller resulted in streaking of printed sheets because of non-uniform surface characteristics of the metering roller which caused a non-uniform film of dampening fluid to be metered onto the surface of the transfer roller.
To accommodate existing press design, metering and transfer rollers have been constructed of diameters generally in a range of approximately 3 to 6 inches. At surface speeds of about 300 feet per minute, films of dampening fluid tended to separate from the surface of the metering roller as a result of centrifugal force. When the metering roller, geared to the transfer roller, was slowed to prevent splashing and slinging of dampening fluid, excessive slippage resulted at the nip between the transfer roller and the form roller which carried the dampening fluid to the lithographic printing system.
In applications where the metering roll was geared to the hydrophilic transfer roller and where a relatively fast hydrophilic transfer roller surface speed was required for printing, such as in a web press, the metering roller slung water to such an extent that experiments were conducted on apparatus wherein the transfer roller and metering roller were geared together to run at a speed ratio of 2:1 thereby substantially reducing the surface speed of the metering roller which carried a thick film of dampening fluid. Slinging of dampening fluid was then stopped but the film of dampening fluid delivered by the transfer roller was of a thickness which required excessive slippage, resulting in excessive emulsification, between the hydrophilic transfer roller and the form roller.